Sam - Unless I'm misunderstanding, if it's taking as long as 40 seconds after you un-mute for the noise to happen, and it continues to happen randomly 5 minutes down the road, it would seem to me it has nothing to do with the mutes. Unfortunately, for you to be able to chase it, find it, and fix it, you're going to have to reproduce it...Consistently. Do you have access to a laptop and DAW? You could run USB or FireWire from the console into the laptop, and record all 32 channels. Once you have the noise recorded, you could go back and check the recording to see if you can narrow it down to any specific input channel(s). If it's not on the inputs, you can rule them out and move on to bus's and outputs. You could also do an overall recording to USB stick. That won't allow you to narrow down by channel, but you could maybe narrow down by what's happening on stage when the noise hits and see if there's any pattern.